The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without payment of any royalties thereon or therefor.
1. Field of the invention
The present invention relates to strong-motion seismology (the recording of large accelerations from large earthquakes in the near and middle-distance fields) and, more particularly, to an improved accelerometer system for use in high spatial density urban arrays for near-real-time mapping of strong shaking due to large earthquakes.
2. Description of the Background
Strong-motion seismology entails the recording of large, linear accelerations from large earthquakes in the near and middle-distance fields. Strong motion seismology uses sensors called accelerometers to record these large-amplitude ground motions and the response of engineered structures to these motions. The recorded large-amplitude seismic waves can be used in researching the fault motions that produced the earthquake, the basin and other xe2x80x9cpath effectsxe2x80x9d modifying the motions between the fault and a given site, the local xe2x80x9csite effectsxe2x80x9d such as local material wave speed, and the responses of the built environment at the site, together hopefully allowing prediction of the patterns of strong shaking from future large earthquakes.
In 1931, Congress allocated funds to the U.S. Coast and Geodetic Survey for development of a strong-motion seismograph (accelerograph), and the implementation and operation of a national strong-motion network. The first U.S. accelerographs were installed in southern California in the summer of 1932. By 1972 the network included 575 accelerographs at permanent stations located throughout the United States and in Central and South America. Responsibility for the network was transferred to the National Oceanic and Atmospheric Administration in 1970, and in 1973 the strong-motion program was absorbed by the U.S. Department of the Interior, U.S. Geological Survey. Today, the U.S. Geological Survey""s National Strong-Motion Program (NSMP) has the primary Federal responsibility for recording each damaging earthquake in the United States. The program maintains a national cooperative instrumentation network, a national data center, and a supporting strong-motion data analyses and research center in support of this responsibility. Indeed, the NSMP counts some 1200 stations that participate in the National Strong-Motion Network (NSMN), and it operates over 900 strong-motion instruments of its own at approximately 628 permanent stations located in 32 States and the Caribbean. The NSMP currently employs two basic types of accelerometers:
(1) state-of-the-art research-grade instruments using macroscopic accelerometers (the most common examples are the Kinemetrics FBA-23(trademark) and EpiSensor(trademark)). These typically cost $1000 or more per axis, with three axes required in a research instrument. Moreover, they are fragile, and easily destroyed by dropping even an inch or two onto a hard surface. They require careful, routine adjustments;
(2) lower grade macroscopic accelerometers which offer relatively poor resolution.
In addition, there are micro-machined accelerometers for seismic applications, for example, the Kinemetrics QDR(trademark) and the Tokyo Gas Co., Ltd., SI Sensor(trademark). The QDR(trademark) has RMS (root mean square) noise levels over DC to 25 Hz of about 2.5 mg (thousandths of one xe2x80x9cgxe2x80x9d, one g being the acceleration due to the Earth""s gravity at its surface). Compared to a xc2x12 g full-scale range, this is 58 dB dynamic range (or even 3 dB less if one were to compare to an xe2x80x9cRMS full scalexe2x80x9d, which would be 2/2 g. The SI sensors cost is in the vicinity of $5000 (complete with a recording and valve-control system). The accelerometer alone would be priced at about $600 for three components and reportedly has a dynamic range of somewhat better than 66 dB.
The severity of earthquake ground shaking varies tremendously over very short distances. For example, FIG. 1a shows the spatial variability of strong ground motion, expressed as the log-normal standard deviation between neighboring stations. Within a distance of as little as 1 km from the nearest station, one knows little more than what can be obtained from an attenuation relation, given only distance from the fault rupture, the geology of the site, and gross source directivity. For example, if some station measures 0.5 g peak ground acceleration (PGA), then at that distance of 1 km, under otherwise identical conditions, the shaking has one chance in three of being under 0.36 g or over 0.70 g, based on the curve shown in FIG. 1b. This large degree of variance over such a short distance can be the difference between moderate and heavy damage. Hence, there are critical needs, both in emergency response and in mitigation, to sample ground shaking densely enough to identify individual neighborhoods suffering localized, strong shaking. Dense sampling would be prohibitive using the foregoing accelerometers, which are either very expensive or suffer from poor performance. Thus, there is a great need for a low-cost, and yet robust high-performance accelerometer that is better suited for a spatially dense network of strong-motion seismographs.
There has been some work on micro-machined accelerometers. See, e.g., Evans, J. R., and J. A. Rogers, Relative performance of several inexpensive accelerometers, U.S. Geol. Surv. Open File Rep., 95-555, 38 pp., 1995, and Evans, J. R., The design and performance of a low-cost strong-motion sensor using the ICS-3028(trademark) micromachined accelerometer, U.S. Geol. Surv. Open File Rep., 98-109, 30 pp. Evers (1998) introduced the ICS-3028(trademark) micromachined accelerometer and suggests a new paradigm in strong-motion seismology where the necessarily few high-grade research instruments are augmented by spatially dense networks of robust, lower-cost instruments. Micro-machined silicon sensors generally have very significant cost and toughness (robustness) advantages over traditional macroscopic sensors. With these, it becomes possible to produce the equivalent of Doppler weather radar, showing earthquake-shaking xe2x80x9cstormsxe2x80x9d and the badly shaken xe2x80x9csquall linesxe2x80x9d within them. This level of detail would benefit Emergency Services, seismologists, structural engineers, and others. The foregoing advantages, however, are often offset by higher instrumental noise (lower dynamic range). It would, therefore, be greatly advantageous to provide a supporting architecture and a new technique for precision temperature calibration and compensation which reduces the instrumental noise (improves the dynamic range), and increases the precision of the ICS-3028(trademark) micromachined accelerometer, and does so with very low power consumption, thereby providing an accelerometer package with cost and performance advantages over existing art.
It is, therefore, an object of the present invention to provide a strong-motion seismological accelerometer system with very robust sensors that are resistant to mechanical shock, spring sag under long exposure to gravity, and other abuses, wear, and tear, resulting in low maintenance costs, and yet which can be manufactured at low cost.
It is another object to preserve the inherent dynamic range of the ICS-3028(trademark) while compensating for it, and doing this at very low power.
It is another object to provide a strong-motion seismological accelerometer system as described above that provides for full calibration and compensation for the effects of temperature on sensor gain and offset.
It is still another object to provide a strong-motion seismological accelerometer system as described above with excellent signal-quality characteristics and low system noise with  greater than 90.3 dB signal-to-noise ratio in the band 0.1 to 35 Hz.
It is yet another object to accomplish all of the foregoing in a configuration that provides for practical manufacturability.
According to the present invention, the above-described and other objects are accomplished with an improved accelerometer system principally for use in high spatial density urban arrays for near-real-time mapping of strong shaking due to large earthquakes. The system relies on a robust accelerometer sensor that includes three accelerometer sensor circuit boards for measuring acceleration along the seismic axes xe2x80x9cVerticalxe2x80x9d (positive up), xe2x80x9cNorthxe2x80x9d, and xe2x80x9cEastxe2x80x9d in that order, which correspond in a right-handed (x,y,z) coordinate system to the axes z, y, and x, respectively. Each of the boards includes a micro-machined accelerometer component with a piezo-resistor bridge and certain compensating resistors. In addition, a main circuit board houses a regulated power supply, amplifiers, and other peripheral circuitry. All of the foregoing circuit boards are mounted on a non-conductive block having at least three substantially orthogonal faces. Thus, the three accelerometer sensor circuit boards are each mounted on a respective orthogonal face of the non-conductive block.
This configuration helps to achieve research-grade xe2x80x9c16-bit resolutionxe2x80x9d (=90.3 dB dynamic range =20log(215)), and yet it is inexpensive to buy and inexpensive to maintain. This makes the present sensor configuration ideally suited to any situation requiring large numbers of instruments, low installation costs, high robustness, low maintenance costs, and near-real-time response. Moreover, the design configuration provides for practical manufacturability, which is no small feat in this context. In particular, the severance of the circuitry into discrete accelerometer sensor boards containing the accelerometer component with piezo-resistor Wheatstone bridges, plus a fourth and separate main circuit board containing the supporting circuitry (amplifiers and anti-alias filters) for all three accelerometer sensors, as well as the orthogonal mounting of the circuit boards on a non-conductive mounting block are integral features toward accomplishing this.
An improved calibration and compensation technique is also disclosed which includes a Manufacturing Calibration Procedure performed in the manufacturing plant, the results being used for Digital Compensation by software after the output of the accelerometer 100 is digitized (a zero noise, high accuracy, low cost method). This dual-calibration and compensation division between passive low-noise analog and active zero-noise digital is another key element of the present accelerometer system, required for preserving the low noise characteristics of the ICS-3028(trademark). In effect, this allows low-cost low-noise high-precision compensation for temperature effects. Overall, the price/performance point achieved by the present invention far exceeds past instrumentation and opens many new markets.